Optical properties of ZnO thin films dispersed with noble metal nanoparticles synthesized by sol-gel method

Zinc Oxide (ZnO) is an inexpensive n-type semiconductor having a direct band gap of 3.3eV with a large exciton binding energy of 60meV. Noble metal nanoparticles show a surface plasmon resonance in the visible region due to collective oscillations of electrons at the surface of metal nanoparticles. The unique features in the composite system of dielectrics-metal nanoparticles have potential applications in optoelectronic devices such as transparent conductive films, solar cells, photocatalysts and so on. In this study, ZnO thin films dispersed with Ag or Au nanoparticles were synthesized using a sol-gel technique. X-ray diffraction peaks of ZnO films exhibited a pattern corresponding to the hexagonal wultzite structure. In the TEM analysis of ZnO-Au composite films, spherical Au nanoparticles were observed within the ZnO crystalline matrix. The distribution of the diameter of Au nanoparticles was centered at around 20nm and broadened with the half width of about 20nm. In the ZnO-Ag composite films, Ag nanoparticles grow larger as the annealing temperature becomes higher and various shape of Ag precipitations like triangular and square plates were observed in ZnO-Ag (50:50) composite films. The optical absorption peaks were observed at 580nm and 410nm due to the surface plasmon resonance of gold and silver nanoparticles, respectively. The absorption spectra were analyzed using a typical effective medium approximation of Maxwell-Garnett model and good fitting was obtained for a ZnO-Au composite film assuming spherical Au nanoparticle. The spectra were discussed relating with the size and shape of the nanoparticles, and the refractive index of the matrix.